Every strap question eventually comes back to one number. Not your wrist size, not the brand of the watch — the lug width. Get it wrong by a single millimetre and the strap either will not go in, or it rattles around once it does.
The good news: it takes about a minute to measure, and you almost certainly own the tools already.
What lug width actually is
The lugs are the four horns sticking out of your watch case — two at the top, two at the bottom. The strap sits between each pair, held by a spring bar. Lug width is the distance between the inside faces of one pair of lugs.
That gap is what your strap has to fill. On almost every modern watch it is a whole, even number between 16 mm and 24 mm, with 20 mm and 22 mm by far the most common. Odd sizes exist — 19 mm and 21 mm turn up on plenty of Seiko and Citizen models — which is exactly why guessing is a bad idea.
How to measure it
- Take the current strap off, or at least push it aside. You need to see bare metal on the inside of both lugs.
- Lay a millimetre ruler across the gap, resting it against the inside face of one lug.
- Read across to the inside face of the other lug. Keep the ruler square to the case, not angled.
- Round to the nearest whole millimetre. If you land on 19.6, the answer is 20.
Digital calipers make this easier and are worth the small outlay if you swap straps often — slide the jaws into the gap, and read off to a tenth of a millimetre. A cheap plastic set is plenty accurate for this.
The mistake almost everyone makes
Do not measure your existing strap. Measure the watch.
Straps stretch, compress and deform with wear, especially leather. A strap that started life at 20 mm can read 19.4 mm after a couple of years on the wrist. Worse, plenty of straps are tapered — wide at the lugs, narrower at the buckle — so measuring at the wrong point gives you a number that was never the lug width to begin with.
If the strap is genuinely all you have, measure it at the very end that meets the case, and treat the result as a starting guess rather than an answer.
What about the buckle end?
On a tapered strap the buckle end is usually 2—4 mm narrower than the lug end. A 20 mm strap often finishes at 16 mm or 18 mm.
This matters the moment you want to change the hardware. If you are fitting a deployant clasp, the size you order is the buckle end, not the lug end — so measure across the strap where the current buckle sits.
Lug width is not the whole story
Lug width tells you whether a strap will fit. Two other numbers decide whether it will suit you:
- Strap length. Usually quoted as two figures, like 75/115 mm — the short side and the long side. A wrist under about 16 cm is happier on a short strap; over 19 cm and you want an extra-long.
- Lug-to-lug distance. The length of the case itself. It has nothing to do with strap fit, but it is the number that decides whether the watch overhangs your wrist.
Once you know your number
Write it down somewhere — you will use it every time you buy a strap. Then have a look through the full strap collection, which spans 16—28 mm across leather, rubber, nylon and steel.
And if you would rather have something built to your exact specification, the bespoke strap builder starts by asking for precisely this measurement, then lets you choose the leather, stitching, edge paint and lining from there.